You have to have a certain post count and or be here a certain length of time before you'll see the "Edit Post" button. But, you'll have a certain amount of time after you make a post to edit the post.
We do this as an "anti spam" anti arguing with other members kind of deal..
You reach the magic number pretty soon.
In the meantime....if its a major correction, just make a correcting post to clarify what you intended to post....